Rechargeable or secondary batteries such as those used in computers, hearing aids, radios, cellular telephones and the like have found wide acceptance as a useful source of energy. The ability to deliver power at appropriate currents and voltages over a long period of time with regular recharging is important in making many of these devices popular and commercially successful.
As more sophisticated electronic equipment is developed that uses its own self-contained energy source, the limitations of the conventional and well known secondary batteries become more important. Size and shape and weight considerations for the power source limit the development of most portable electronic products, at least forcing compromises in performance or life or range of use.
Lithium batteries have added greatly in the development of newer electronic devices because lithium batteries have a high energy to weight and/or volume ratio. Lithium batteries have been particularly important as primary batteries, which are those that are capable of producing electrical current by an electrochemical reaction in the discharge mode one or at best two or three times. Most lithium batteries are not rechargeable, not operating in the secondary mode.
Portable devices such as computers, camcorders, telephones and the like use nickel-cadmium or nickel-metal hydride batteries as the primary power source and a small lithium battery as a backup power source for memory protection and the like, usually in the primary battery configuration.
Attempts to make secondary lithium batteries using lithium metal as the negative electrode's active material have been made, resulting in the formation of pyrophoric, finely divided metal, inefficient utilization and explosiveness due to electrical short circuits. While attempts to continue the use of the lithium metal or its alloys are continuing, the present state of the art calls for the use of special materials to contain the lithium in an ionic form; hence, the term "lithium-ion" is applied to a new emerging class of secondary cells and batteries.
Presently there are several lithium ion technologies that have been proposed, in which various negative electrode and positive electrode materials are employed. All are high voltage, nominally around 4.0 volts depending upon the specific electrochemistry used.
The negative electrode of lithium ion batteries is generally carbon in some form, such as petroleum coke or graphite, with graphite being preferred due to the ability to provide greater capacity at higher potentials than petroleum coke in particular or disordered carbons, in general. The positive electrode materials are most often transition metal oxide materials such as those using cobalt, nickel or manganese. There are three positive electrode materials presently used in lithium ion batteries, and all are similar in ability but slightly different in operating voltages. These materials are LiNiO.sub.2, LiCoO.sub.2 and LiMn.sub.2 O.sub.4 in specific forms because they are capable of being manufactured chemically in a fully lithiated state. Because of this, cells are manufactured in the discharged state with the positive electrode material acting as the reservoir of lithium ions needed for cell reactions, avoiding the use of highly active lithium metal.
It has been understood that there must be a quantity of lithium metal incorporated into a cell using a V.sub.2 O.sub.5 electrode to provide a source of lithium ions. These cells include lithium metal foil laminated with carbon as the negative electrode. The principle difficulty that has been encountered in the development of the V.sub.2 O.sub.5 lithium batteries such as those that can be operated as secondary batteries is, not surprisingly, the lithium metal. Lithium use metal increases costs, decreases safety if only for the presence of residual finely divided lithium metal in discarded cells, and makes overall assembly more difficult if not more costly. One such lithium secondary battery is shown in U.S. Pat. No. 3,929,504. In that cell, the negative electrode comprises a lithium metal ribbon pressed on to an expanded copper metal grid. While the battery is effective over a large number of recharge cycles, it is not without the inherent danger of any cell containing lithium metal.
The prior art has not at this point developed an electrochemical cell configuration that uses a lithium metal free V.sub.2 O.sub.5 secondary cell. At the present time, useful lithiated V.sub.2 O.sub.5 is not available and this material has, essentially, not been found to exist alone in nature. LiV.sub.2 O.sub.5 is not available and there is no reported method for its manufacture.
Simon U.S. Pat. No. 5,232,795 discloses a rechargeable cell having a solid electrolyte. The cell comprises a graphite negative electrode, a lithium salt in a polymer as an electrolyte, and a cathode including, inter alia, LiV.sub.2 O.sub.5. There is no suggestion as to where the LiV.sub.2 O.sub.5 can be obtained, in contrast with the sole example in which LiCoO2 is shown and referenced as being sold by Aldrich. Simon fails to enable one to make such a cell as no known source of suitable LiV.sub.2 O.sub.5 presently exists.
Labat et al U.S. Pat. No. 5,219,677 discloses a rechargeable cell having a cathode based on V.sub.2 O.sub.5. The cell includes a lithium or lithium alloy anode, an electrolyte having a lithium salt in a nonaqueous solvent, and a cathode based on vanadium oxide. Labat et al teaches that the cathodic version of LiV.sub.2 O.sub.5 in their invention is formed by discharging a cell having a V.sub.2 O.sub.5 cathode and lithium anode such that the cell is charged to 3.8 volts. Discharging was stopped at 2.8 volts in what Labat et al terms a prior art cell and a second cell in accordance with the Labat et al invention was discharged to 2.0 volts. An advantage is disclosed for the cell that was discharged to a greater extent.
Labat et al clearly teach that they form a preferred form of cathodic material that is designated gamma LiV.sub.2 O.sub.5. Cathodes that are discharged to 2.8 volts are shown to be inferior. In any event, Labat et al does not disclose a method of producing LiV.sub.2 O.sub.5 for use in cells that do not include lithium metal, either in metal or alloy form. Labat et al further does not disclose that an effective cathode material including LiV.sub.2 O.sub.5 may be prepared unless it is initially discharged to about 2.0 volts as described in the reference. Finally, Labat et al does not disclose that LiV.sub.2 O.sub.5 may be used in cells that do not have lithium metal or metal alloy.
Mural et al. U.S. Pat. No. 5,316,875 teaches that electrodes for nonaqueous batteries may be made using several lithium containing positive electrodes. Specifically disclosed are LiMn.sub.2 O.sub.4, LiMnO.sub.2, LiCoO.sub.2, LiFeO.sub.2 and gamma LiV.sub.2 O.sub.5. The LiV.sub.2 O.sub.5 form is identified in Mural et al. more precisely as being Li.sub.x V.sub.2 O.sub.5 where x is stated to be greater than 1. It is prepared, as set forth in Embodiment One of that patent by mixing Li.sub.2 CO.sub.3 and V.sub.2 O.sub.5 in a 1:2 mol ratio followed by heating at 700.degree. C.
The gamma form of LiV.sub.2 O.sub.5 is shown to be effective in certain cells, but is made more effective by dipping the positive electrode in a lithium-adding material such as butyllithium, phenyllithium or naphthyllithium. This secondary step after formation of the basic gamma LiV.sub.2 O.sub.5 material is accomplished by dipping the material in a hexane solution of butyllithium to form Li.sub.1.15 V.sub.2 O.sub.5. No X-ray diffraction pattern is shown for this material even though there is such a pattern for the manganese version of the Mural et al. positive electrodes.
It would be of advantage to the art if LiV.sub.2 O.sub.5 were available as a positive electrode material for use in electrochemical cells without the presence of lithium metal in any form.
Accordingly, it is an object of the present invention to provide an electrochemical cell that employs V.sub.2 O.sub.5 in a lithiated form without the presence of lithium metal in the cell.
Another object of this invention is to provide a positive electrode which is formed from LiV.sub.2 O.sub.5.
Other objects will appear hereinafter.